The present invention generally relates to an internal combustion engine (hereinbelow, referred to as an "engine") and more particularly, to an intake arrangement for the engine, in which a combustion chamber of each of the cylinders is provided with a plurality of, for example, two intake ports each having an intake valve for opening or closing each of the intake ports such that at least one of the intake ports is selectively used in accordance with a load applied to the engine.
Conventionally, in intake arrangements for reciprocating engines equipped with a plurality of cylinders each having a combustion chamber and a cylinder head, it is known that two intake ports having a substantially identical bore area are provided on the combustion chamber so as to secure a large intake area and an intake passage formed in the cylinder head is connected to the intake ports at a small angle with an axis of the combustion chamber so as to cause intake gases to flow into the combustion chamber substantially along the axis of the combustion chamber such that the engines can yield high outputs by maximizing charging efficiencies of the engines. The above-known two-port type intake arrangements are advantageous in that it becomes possible to obtain high outputs from the engines under high load engine operating conditions, but have such inconveniences that low flow velocity of the intake gases, inferior combustibility of an air-fuel mixture, uneconomical fuel consumption and an increase of harmful compositions in the exhaust gas are incurred under low load engine operating conditions.
In order to eliminate the above described drawbacks, Japanese Patent Laid-Open Publication No. 44419/1981 (Tokkaisho 56-44419), for example, discloses an intake arrangement for an engine, in which a first branch intake passage (for low loads) and a second branch intake passage (for high loads) provided with a shutter valve are led from a main intake passage so as to be, respectively, connected to two intake ports formed on the cylinder head and the shutter valve of the second branch intake passage is closed under the low load engine operating conditions such that intake is performed by using only the first branch intake passage. More specifically, the main intake passage is provided with a throttle valve and the throttle valve is operatively associated with the shutter valve such that the shutter valve is opened upon opening of the throttle valve beyond a predetermined opening degree. Namely, under the low load engine operating conditions, since the second branch intake passage is closed by the shutter valve, the intake gases are supplied at a relatively high flow velocity through only the first branch intake passage. Meanwhile, under the high load engine operating conditions, since the intake gases are supplied through both the first and second branch intake passages, a large charging amount of the intake gases can be secured.
However, since this prior art intake arrangement is originally designed to secure a sufficiently large amount of the intake gases under the high load engine operating conditions, each of the first and second branch intake passages has a rather large cross-sectional area. Accordingly, even if only the first branch intake passage is used under the low load engine operating conditions, it is impossible to increase a flow velocity of the intake gases sufficiently under such extremely low load engine operating conditions as idling, so that it becomes impossible to effectively produce swirling motions of an air-fuel mixture in the combustion chamber, which swirling motions are indispensable for improvement of combustibility of the air-fuel mixture. It may be imagined that such a problem can be solved by simply reducing the cross-sectional area of the first branch intake passage. However, since the first and second branch intake passages are, respectively, connected to the intake ports substantially along the axis of the combustion chamber so as to increase an output of the engine as described above, effective swirling motions of the air-fuel mixture along a circumferential direction of the combustion chamber cannot be produced even if a flow velocity of the air-fuel mixture in the combustion chamber is increased by increasing the flow velocity of the intake gases, thereby resulting in rapid reduction of the flow velocity of the air-fuel mixture in a compression stroke. Meanwhile, in the case where the cross-sectional area of the first branch intake passage is reduced extremely so as to increase the flow velocity of the intake gases as high as possible, a load range usable in the engine is restricted accordingly, so that it becomes necessary to supply the intake gases also from the second branch intake passage by opening the shutter valve even under relatively low load engine operating conditions. At this time, since the two intake ports are formed symmetrically with a horizontal center line of the combustion chamber, the intake gases drawn from the first branch intake passage are caused to collide with those drawn from the second branch intake passage, so that swirling motions of the air-fuel mixture are impaired or almost extinguished and thus, excellent combustibility of the air-fuel mixture based on its vigorous swirling motions cannot be obtained.